The Rotation of Low-Mass Pre-Main-Sequence Stars

Major photometric monitoring campaigns of star-forming regions in the past decade have provided rich rotation period distributions of pre-main-sequence stars. The rotation periods span more than an order of magnitude in period, with most falling between 1 and 10 days. Thus the broad rotation period distributions found in 100 Myr clusters are already established by an age of 1 Myr. The most rapidly rotating stars are within a factor of 2-3 of their critical velocities; if angular momentum is conserved as they evolve to the ZAMS, these stars may come to exceed their critical velocities. Extensive efforts have been made to find connections between stellar rotation and the presence of protostellar disks; at best only a weak correlation has been found in the largest samples. Magnetic disk-locking is a theoretically attractive mechanism for angular momentum evolution of young stars, but the links between theoretical predictions and observational evidence remain ambiguous. Detailed observational and theoretical studies of the magnetospheric environments will provide better insight into the processes of pre-main-sequence stellar angular momentum evolution. 1. The Angular Momentum Problem The specific angular momenta of the interstellar clouds from which stars form are significantly higher that that of the Sun, a fact that has been recognized for many decades. The numbers shown in Table 1 give scale to the “angular momentum problem”, with dense molecular cores having six orders of magnitude higher specific angular momentum than the Sun. In many, and perhaps all, cases the clouds partially solve this problem by depositing much of their angular momentum in the orbital motions of stellar and planetary companions. As shown by Vogel & Kuhi (1981) and Hartmann et al. (1986), most of the angular momentum problem has been solved by the pre-main-sequence (PMS) stage of evolution. Nonetheless, a factor 10-100 reduction in specific angular momentum is still required to derive the solar rotation at 4.5 Gyr, and indeed to produce the rotation of main-sequence solar-mass stars at 100 Myr. Table 1. Specific angular momenta (cms) Dense molecular cores 10 Wide binary stars 10 Pre-main-sequence stars 10